This invention relates to a system for recovering drain of feed-water heaters in a generating plant, and, more particularly, to a system for recovering drain which is suitable for controlling a content of dissolved oxygen contained in feed water of, for example, a nuclear reactor.
In the condensation system of a conventional generating plant such as, for example, a BWR nuclear power plant, all cascade drain of feed-water heating apparatuses is recovered into a condenser and all water is degassed by a condenser and fed to a nuclear reactor through a condensed water processing apparatus so as to improve a quality of the water. However, the capacity of a low pressure condensation pump and the capacity of the condensed water processing apparatus must be increased.
Since BWR nucular power plants include a degassing apparatus, special considerations must be taken for degassing cascade drain.
While Japanese Laid Open Patent Application 7903/81 proposes a tank in a drain line, the proposal contained therein does not address the problems encountered in regulating a control of a content of dissolved oxygen contained in the condensed water.
The aim underlying the present invention essentially resides in providing a system for recovering drain which effects degassing cascade drain from feed-water heating apparatuses in a drain tank and facilitates controlling degassing ability in order to maintain the content of dissolved oxygen in condensed water within a range (normally 20-50 ppb) required as a system from the standpoint of resistance to corrosion.
There are roughly three approaches for reducing a content of the dissolved oxygen in recovered drain; namely, degassing by heating, separating by mechanical stirring, and venting. Of the above noted three approaches, degassing by heating and venting can be relatively easily controlled. It has also been determined that, if the content of the dissolved oxygen is extremely small, it does more harm than good from the standpoint of resistance to corrosion. Therefore, when the content of dissolved oxygen in the recovered drain is too small, another approach resides in increasing the oxygen content by injecting oxygen directly or leaking air. This last approach is, as with the degassing by heating and venting approaches, also easily controllable and a system which facilitates controlling the content of the dissolved oxygen optionally can be constituted by a combination of the above-noted approaches.
To this end, the present invention provides a system for recovering drain which recovers two or more flows or drain of different temperatures, which is applied to a generating plant provided with feed-water heaters in which drain is recovered into a condensation system downstream from a condenser through a drain tank by a drain pump and which comprises a drain tank, an inlet for drain of high temperature, which is at high temperature and contains much flush steam, and a low temperature drain inlet. The high temperature drain inlet is positioned at a lower part of the drain tank and the low temperature drain inlet is positioned above the former.